We identified a population of primitive cells that is co-selected when MSC are purified from human or rodent marrow that have, at the single cell level, multilineage differentiation and extensive proliferation potential, which we named Multipotent Adult Progenitor Cell or MAPC. MAPC differentiate in vitro into most mesodermal cell types as well as cells with neuroectodermal and endodermal features. Using retroviral marking we have preliminary evidence that multi-lineage differentiation is derived from single cells. We have also initial evidence that MAPC engraft in vivo, persist for 2+ months, and may be able to differentiate into tissue specific cells in response to local "cues", even though single cell origin of the engrafted cells that differentiate in multiple tissue specific cells has not yet been proven. Our finding that MAPC differentiate into cells with not only mesodermal but also endodermal and neuroectodermal characteristics in vitro and engraft in multiple tissues when transplanted, is consistent with recent publications showing that marrow derived cells can acquire markers of muscle, endothelium, liver and neuroectoderm in vivo. If proven further, such previously unrecognized plasticity of post-natal stem cells opens the exciting possibility that post-natal stem cells from easily accessible sources such as marrow could be used to treat a number of degenerative or inherited disorders. However, many questions remain, including 1) What is the nature of the stem cell with plasticity? 2) Is plasticity due to co-existence of multiple tissue specific stem cells in the marrow, or can a single cell differentiate into most tissue cells? 3) What is the mechanism underlying the commitment and differentiation of presumed multipotent marrow derived mesenchymal stem cells to non-mesenchymal lineages? 4) Can presumed multipotent mesenchymal stem cells repopulate organs other than marrow? 5) Will high level engraftment and differentiation in vivo require that these multipotent mesenchymal stem cells are induced to commit to the host organ tissue prior to transplantation? 6) Can multipotent mesenchymal stem cells be used in stead of organ specific stem cells to repopulate an organ in vivo? We propose the following specific aims: SA1: To demonstrate that multipotent cells exist in post-natal marrow that, at the single cell level, can differentiate into cells of mesodermal and neuroectodermal lineage. Specifically, we will demonstrate using in vitro clonal assays that single cells from murine marrow can proliferate without senescence and can at the single cell level differentiate into cells of mesodermal and neuroectodermal lineage. We also plan to assess the frequency of cells with these characteristics in MSC and develop methods to purify such cells. SA2: Can MAPC differentiate into functioning neurons in vitro and what are the intermediary cell types that are generated? We plan to use neuroectodermal differentiation from MAPC to evaluate the plasticity of somatic stem cells. We will examine the degree of differentiation that can be obtained, and also examine the mechanism of commitment and differentiation of MAPC by comparing the nature and potential of intermediary progenitors and precursors that are generated from MAPC with that of brain derived neuroprogenitors. SA3: Can MAPC differentiate into functioning neurons in vivo? We will evaluate whether multipotent marrow derived stem cells or their neuroectodermal committed progeny repopulate the host brain in vivo and acquire region specific differentiation characteristics in response to local cues.